A conventional pad for an air bag device is disclosed in Japanese Unexamined Patent Publication No. 2-171364. This type is characterized as being comprised of a single-layered structure made of a thermoplastic elastomer having an outer wall covering a folded air bag.
The outer wall of the pad has a portion that is supposed to break at the time the air bag expands (hereinafter referred to as "the breaking portion") and an ordinary portion excluding such breaking portion. The breaking portion is substantially H-shaped, as viewed in a vertical direction, and thin-walled. The ordinary portion has a thickness larger than the thickness of the breaking portion so that the air bag can expand smoothly from the pad upon breakage of the outer wall which breaks like the opening of a pair of French doors.
To arrange a decorative mark or the like in the middle of the outer wall of the pad, straight line portions and a circuitously curved U-shaped portion are provided on a horizontal rod portion of the substantially H-shape in the breaking portion. The straight line portions, that are respectively connected to two vertical rod portions on both left and right sides of the substantially H-shape and the U-shaped portion that is interposed between the straight line portions, are formed in order to prevent scattering of the decorative mark upon breakage of the ceiling wall subsequent to the expansion of the air bag. This is disclosed in Japanese Unexamined Utility Model Publication No. 1-81350.
However, when the U-shaped portion is formed on the horizontal rod .portion in the breaking portion, a single layered type with no embedded insert pad is not likely to break along the U-shaped portion, especially when made of a soft thermoplastic elastomer. This is because breakage of the breaking portion, in association with the expansion of the air bag, tends to proceed in a straight line.
That is, when breakage of the pad starts at the middle of the U-shaped portion, the breakage of the pad proceeds, in some cases, straight toward the vicinities of intersections between the vertical rod portions, on both left and right sides, and the straight line portions from the middle of the U-shaped portion without going along the U-shape.
In this case, as disclosed in Japanese Unexamined Utility Model Publication No. 1-83652, it is conceivable to make the thickness of the ordinary portion, that extends all along the breaking portion, thicker than the rest of the ordinary portion.
However, if the thickness of the ordinary portion that extends all along the breaking portion is thicker than the rest of the ordinary portion, the vicinities of the intersections between the vertical rod portions on both left and right sides of the breaking portion and the straight line portions serve as the corner portions of two French doors that open on hinges while interposing the horizontal rod portion therebetween at the time the air bag expands after the breakage of the breaking portion. Such corner portions, however, are not soft to the touch because they are thicker and will not yield as the thinner walled areas.